The present invention is directed to a process for constructing a spectrophotometer from a monolithic substrate.
Spectrophotometers are optical instruments which separate optical signals according to their wavelengths. They have broad applications including color identification in flat panel displays or electronic cameras, color control for xerographic printing, optical spectroscopy for chemical analysis, environmental monitoring, and process controls which are related to color identification. Up to date, all commercial spectrophotometers tend to be of rather large size because they are formed by assembling bulky optical elements, mechanical parts, detectors, and microelectronic chips into a system. This current assembly process needs high precision and is labor intensive, keeping the cost of conventional bench top spectrophotometers from being affordable. There are many additional applications of interest which would arise if spectrophotometer were of significantly lower cost, lighter weight, smaller size, rugged, and incorporated signal processing capability in the instrument. In xerographic printing, a spectrophotometer is a key component in a closed-loop color control system which will enable the printers to generate reproducible color images in a networked environment. The development of a compact, low cost spectrophotometer is thus important ox in realizing high performance printing systems.
With the advance of micromachining technology, it is now possible to build various microstructures, movable mechanical components, micro optical elements, including free-space, out-of-plane lenses and gratings, sensors, and electronic circuits on silicon chips using modified IC processes that are able to produce thousands of these devices in batch on silicon wafers. Over the past decade, much effort has been devoted to the development of micro spectrophotometers using MEMS technology. However, none of these initial efforts were successful, partly because of the technical difficulty associated with the integration of high-precision optical elements and photodetectors in a system. The fabrication of these spectrophotometers needs either a high-precision wafer-to-wafer bonding or special thin film deposition processes for building microgratings or dispersive waveguides on a chip. The alignment of these optical elements with the photodetectors is very critical. Any misalignment in the scale of as small as one micro meter will result in significant deviation in device performance. As a result, none of these prototype devices has been commercialized thus far.
This spectrophotometer incorporates concave gratings, photo diode array, and signal processing circuitry on a silicon substrate and is significantly reduced in size, weight, and cost. On the spectrophotometer chip, the concave gratings for optical wave separation may be defined using a dry etch on either crystal silicon or polyimide. The optical elements and the photo diode array may be defined using photolithography on the same silicon substrate, eliminating the complicated alignment and assembling processes which are generally required for fabrication of conventional spectrophotometers. In order to effectively sense the light reflected from the gratings, the photo diode array is built on a suspended silicon bridge which is bent 90 degrees from the wafer surface. The integration of signal processing circuitry further enhances its function and improves the signal-to-noise ratio, resulting in a high resolution spectrum analysis system.
In the present invention, a fully monolithic spectrophotometer on silicon using MEMS technology is described. This spectrophotometer incorporates concave gratings, a photo diode array, and signal processing circuitry on a silicon substrate and is drastically reduced in size and intricacy. On this chip, the concave gratings for optical wave separation is defined using a dry etch on either crystal silicon or polyimide. The optical elements and the photo diode array are defined using photolithography on the same silicon substrate, eliminating the complicated alignment and assembling processes which are generally required for fabrication of conventional spectrophotometers. In order to orthogonally intersect the light reflected from the gratings, the photo-diode array is built on a suspended silicon bridge which is bent 90xc2x0 from the wafer surface. In this way, the dispersed wave signals, which on this device are designed to propagate along the silicon wafer surface, can be very efficiently sensed by the photodiodes. On this chip, CMOS circuitry and the photodiode array are built at the same time using the same process such that output signals from the photodiodes can be amplified and multiplexed on-chip, decreasing noise pick-up, and allowing conveyance of output signals through a common data bus. This monolithic structure results in a compact spectrophotometer of significantly reduced size and weight. Its cost will also be lowered from the current commercial products because a simplified fabrication process is used